The invention concerns a method for converting a cryostat configuration comprising a room temperature vacuum container containing a first container with a liquid helium bath, the operating temperature of which is kept below 5K by means of helium evaporation, wherein the room temperature vacuum container also encloses a second container which is filled with liquid nitrogen for thermally shielding the first container and can be kept at an operating temperature of between 75 and 80K by means of nitrogen evaporation.
The structure and field of use of a configuration of this type, wherein both containers are arranged inside a room temperature vacuum container, is disclosed e.g. in U.S. Pat. No. 5,267,445.
A cryostat configuration of this type is used, in particular, for cooling superconducting magnet coils. Magnet coils of this type are used i.a. for NMR (nuclear magnetic resonance) measurements. NMR spectroscopy is a powerful method of instrumental analysis. RF (radio frequency) pulses are thereby irradiated into a test sample located in a strong static magnetic field and the RF reaction of the test sample is measured. The relevant information is integrally obtained over a certain area of the test sample, the so-called active volume.
The high magnetic fields that are required for this purpose are generated by superconducting magnet coils which are advantageously operated in liquid helium. The magnet coil and the liquid helium are located in a first container. The temperature of this container remains constant due to continuous evaporation of helium. One or more radiation shields may additionally be arranged around this container. A ring-shaped second container is arranged between these radiation shields and the outer room temperature vacuum container. The second container is filled with liquid nitrogen whose temperature is kept constant at approximately 77K by means of continuous evaporation of nitrogen. A configuration of this type is illustrated in FIG. 8. This structure minimizes the heat input into the first container caused by radiation heat, thereby minimizing, in particular, the evaporation rate of helium in the first container such that helium must be refilled typically only every couple of months.
In view of this passive cooling by means of the evaporating cryogenic helium and nitrogen, liquid helium and nitrogen must always be refilled within certain time intervals. Liquid nitrogen must be refilled within considerably shorter time intervals of one to two weeks.
One substantial disadvantage of this configuration is that handling of the utilized cryogens is complicated and for this reason requires specially trained staff. Moreover, refilling also necessitates undesired interruption of measurements in the cooled apparatus, which is generally undesirable. The dependence on the supply of liquid cryogens is also especially problematic for locations where no optimum infrastructure is provided such as e.g. in developing countries (e.g. India, African countries etc.). Future price increases or shortages of cryogens will also render this type of cooling quite expensive.
The availability of mechanical refrigerators that can achieve temperatures below 4K resulted in the development of further cooling possibilities in order to reduce the dependence on liquid helium and liquid nitrogen. In addition to cooling exclusively by supplying cryogens, there are further prior art cooling variants:                cooling circuits with gaseous helium for pre-cooling or operating superconducting magnets (see for example US 2007245749 A1, EP 0 398 156 B1, WO 95 01 539 A1)        cooling circuits for cooling radiation shields or reliquefaction of nitrogen gas (e.g. in EP 1 655 616 B1)        installation of various cryocoolers into superconducting magnet systems for reliquefaction of cryogens or for cooling of radiation shields (described in EP 0 905 436 B1, U.S. Pat. No. 5,563,566 A1 or U.S. Pat. No. 5,613,367 A1)        external reliquefaction of the cryogens (e.g. in EP 1 628 089 A3)        
The above-described conventional configurations for preventing or reducing consumption of liquid cryogens are all based on the assumption that the cooling circuit is either temporary or is permanently installed on the system right from the start. Configurations for subsequent installation (e.g. in EP 1 655 616 B1, EP 1 628 089 A3) strive for reliquefaction of the evaporating cryogens and therefore for a loss-free system.
This external reliquefaction of the evaporating helium and nitrogen is disadvantageous in that the cooling device must be arranged spatially above the cryostat configuration and requires an increased room height, which is problematic in many laboratories. Experiments for measuring magnetic resonance require e.g. an extremely low-vibration environment. Damping of vibrations generated during operation is correspondingly difficult due to the proximity between the cooling device and the cryostat configuration.
Alternatively, the evaporating cryogens could also be collected outside of the cryostat configuration and be reliquefied by means of a separate cryosystem. Such a system is offered e.g. by the company Cryomech (“Liquid Helium Plants”) but is disadvantageous in that only helium is reliquefied and must be transferred back into the cryostat configuration within relatively short time periods.
In contrast thereto, it would be advantageous in terms of a retrofit for existing cryostat configurations which utilize both liquid helium and also liquid nitrogen e.g. for cooling a superconducting coil, to completely avoid use of liquid nitrogen by means of a retrofittable system and to moreover also considerably reduce the evaporation rate of liquid helium without having to reliquefy the cryogens that are used.
It is therefore the underlying purpose of the present invention to design a method for converting a cryostat configuration comprising the above defined features with as simple technical means as possible such that the above-mentioned disadvantages of prior art are largely prevented, thereby at least omitting use of liquid nitrogen. A further object of the present invention is to substantially reduce helium consumption of the cryostat configuration and moreover enable continuous operation over a long time period with minimum mechanical disturbances.